The overall objective of the research in this proposal is to understand how peptide neurotransmitters affect the output of neuronal circuits and their associated muscles. The results from the research in this proposal will provide information on how neuropeptides influence the normal function of the nervous system and the generation of specific behaviors. Studies of neuropeptides in invertebrates are likely to identify general principles by which these substances act in all animal groups. The leech, as well as other invertebrates, provide several advantages for this type of research. First, the relatively large size of many invertebrate neurons and muscle cells facilitates the study of individual cells. Second, because of the relatively simple organization of invertebrate nervous systems, individual neurons of known function can be identified and studied in each and every preparation. The research described in this proposal will examine the effects of neuropeptides on the neurons and musculature which produce feeding movements in the leech. The neuropeptides in these studies will include peptides which have been identified as being involved in regulating gut motility and/or satiety in either invertebrate or vertebrate preparations. The effect of various neuropeptides on contractions of the feeding musculature will be examined. Those neuropeptides which modify contractions of the feeding musculature will be localized to specific neurons using immunocytochemistry. Neurons involved in generating feeding movements will be identified using standard electrophysiological techniques. In addition, some of the synaptic interactions involved in generating feeding movements will be characterized. When an immunoreactive soma appears to have a similar position as a physiologically identified neuron; a determination will be made as to if these are the same cell. First, the cell will be physiologically identified and then injected with the dye Lucifer Yellow. The ganglion will then be processed for immunocytochemistry. If the dyeinjected cell is also immunoreactive, this will indicate the presence of a particular type of peptide in the identified cell; thus, allowing the tentative identification of that neuron's peptide neurotransmitter. This will open the way for studies on the function of that neurotransmitter at specific synapses involved in the generation of a behavior.